Assessment of blood-brain barrier passage of flumazenil in pharmacoresistant patients with temporal lobe epilepsy

Resistance to current drug therapy is an issue for approximately 30% for all
people who develop epilepsy. Consequently, there is a pressing need to develop
new and more effective treatments.
P-glycoprotein (P-gp) is an efflux transporter (member of the multi-drug
resistance (MDR)- family), which is located at the blood-brain barrier (BBB)
and transports substrates (including multiple CNS drugs) from the brain to
blood and cerebrospinal fluid. Overexpression of P-gp is thought to be an
important mechanism of pharmacoresistance in epilepsy. Various invasive
techniques used in animal studies of epilepsy have shown upregulation of P-gp.
At present upregulation of P-gp in refractory patients can only be confirmed by
examining post-mortem or surgically removed brain tissue. Therefore the
availability of non-invasive imaging methods that would allow the assessment of
the distribution and function of P-gp in the brain is of vital importance.
Currently only [11C]verapamil is available to assess P-gp function by using
positron emission tomography (PET), but is not an ideal ligand to assess P-gp
expression. Novel imaging probes, which are markers for the function of P-gp
need to be evaluated. Such a probe could then be used to establish a
non-invasive molecular imaging-based tool which will allow evaluation of the
role of P-gp for pathophysiology and treatment response in epilepsy and other
major CNS diseases, using the established imaging techniques.
This project concerns a sub-project of EURIPIDES (European Research initiative
to develop Imaging Probes for early In-vivo Diagnosis and Evaluation of
response to therapeutic Substances). One of the important goals of the project
is to quantify to what extent P-gp upregulation affects the binding of an
established PET ligand known to be a P-gp substrate, namely [11C]-flumazenil.
Flumazenil is a ligand that binds to the GABAA-receptor, but has no agonistic
or antagonistic actions on this receptor. Labeled with [11C], flumazenil is
frequently used for PET scanning in epilepsy patients to assess changes in
GABAA-receptor density and to determine focus localization prior to resective
surgery. There is circumstantial evidence from animal and in vitro studies that
flumazenil is a substrate for P-gp. If this is indeed the case, changes in P-gp
expression or functionality would compromise the interpretation of
GABAA-receptor binding data.

(1) To asses the influence of P-gp function at the BBB on flumazenil binding to
the GABAA-receptor in pharmacoresistant patients with temporal lobe epilepsy
(TLE), using PET and [11C]flumazenil as a ligand and tariquidar as P-gp
blocker.

(2) If we conclude, after analysing the data of these twelve patients, that
flumazenil is a substrate for P-gp, twelve healthy volunteers will be recruited
to asses the influence of P-gp function at the BBB on flumazenil binding to the
GABAA-receptor in this group, to allow quantification of the upregulation of
P-gp by TLE.

(3) To study the effect of tariquidar on CBF.

(4) If there is an effect of tariquidar on CBF, we will study the relation
between changes in CBF and [11C]flumazenil uptake in the brain.

Multi-centre proof of concept study in humans.

Risks associated with participation in this study are related to

1) Radiation exposure
The radiation exposure of 1100MBq [15O]water and 370 MBq of [11C]flumazenil is
0.5 milliSievert (mSv) and approximately 2.5mSv respectively. Therefore, each
patient will receive a total radiation dose of 6mSv. For comparison, the
natural background radiation dose in the Netherlands gives an annual dose of 2
- 2.5 mSv. Thus, the total radiation exposure of the total PET procedure is
within an acceptable range. In case of previous exposure to radioactivity,
subjects will be eligible if the yearly cumulative dose due to exposure to
radiation remains below 10 mSv.

2) Idiosyncratic reaction to the tracer or to the P-gp blocker
The injected mass of [11C]flumazenil in this study is negligible.
[11C]flumazenil is a radiotracer that has routinely been used in humans
(patient care). Side effects have never been reported at the tracer doses used
in PET studies. Tariquidar will be given at a dose of 2 mg/kg, equivalent to
what has been given in clinical trials. Tariquidar may, at high doses,
transiently cause a decrease in blood pressure. Furthermore it may cause
haemolysis and change in transaminase activity. For this reason a physician
will constantly be present during and after each injection of the radiotracer
and the injection of tariquidar. Blood pressure and heart rate will be measured
(each 15 minutes) during tariquidar administration and during the subsequent
PET scan.A physician will be present during each injection of the radiotracer
and the injection of tariquidar. Blood pressure and heart rate will be
measured (each 15 minutes) during tariquidar administration and during the
second PET scan.

3) Intravenous and intra-arterial cannulation
There is a very small risk of infection and bleeding associated with
intravenous and intra-arterial catheters, which are prevented by proper
techniques.

4) Blood sampling.
Adverse effects of blood sampling will be minimised by exclusion of subjects
with low haemoglobin levels (Hb must be > 8 mmol / litre at the time of the
scan for males and be > 7 mmol / litre for females). No more than 500ml blood
will be withdrawn during the total PET procedure and screening. Subjects are
excluded if 3 months before the PET procedure substantial blood loss or a blood
donation has occurred. Subjects are advised not to give blood until 3 months
after the scan has been completed.

5) Discomfort during scanning
It may be uncomfortable to lie motionless in the cameras (both PET and MRI) and
it may cause some subjects to feel anxious. Subjects will be made acquainted
with the surroundings beforehand. Our staff will be available to provide
support, reduce anxiety, optimise the comfort of the subject and remove the
subject from the scanner if requested.